1. Field of the Invention
The present invention relates to an image capturing system and a computer readable recording medium for recording an image processing program which performs noise reduction processing on an image signal acquired from an image capturing unit.
2. Description of the Related Art
An image capturing unit comprises an image capturing device, and an analog circuit, an A/D converter and so forth, which are provided as components associated with the image capturing device. In general, an image signal acquired by an image capturing unit includes noise components. These noise components are roughly classified into fixed pattern noise components and random noise components.
The aforementioned fixed pattern noise is noise that occurs principally due to the image capturing device, typical examples of which include noise due to defective pixels.
On the other hand, random noise occurs in the image capturing device and the analog circuit, and has noise characteristics close to white noise characteristics.
With regard to the latter noise, i.e., random noise, a technique has been disclosed in Japanese Patent Application Laid-Open Publication No. 2001-157057, for example, in which the noise amount N is represented as a function, i.e., as N=abcD, using constant terms a, b, and c, which are provided as static terms, and a signal level D which is a signal level converted to a density level. The noise amount N with respect to the signal level D is estimated using this function, and the frequency properties of the filtering are controlled based upon the noise amount N thus estimated, thereby providing adaptive noise reduction processing suitable for the signal level.
Also, a technique is described in Japanese Patent Application Laid-Open Publication No. 2006-23959, in which the noise amount is estimated in increments of blocks based upon a noise model, and noise reduction processing is controlled in increments of blocks. Furthermore, a technique is described in the above-mentioned Publication, in which a region which is similar to a target pixel is selected in each block, and the average value calculated with respect to the region thus selected is used to estimate the noise amount. This allows space variant noise reduction processing to be performed without involving the effects of the edge region, thereby providing a high-quality image signal.
Furthermore, Japanese Patent Application Laid-Open Publication No. 2006-246080 describes an edge preserving filtering technique called bilateral filter employing a combination of smoothing processing and edge enhancement processing. This provides an image with reduced noise components while suppressing degradation of the edge components.
In recent years, a technique has been proposed in which an image that has degraded due to degrading factors such as noise is restored by calculating a diffusion image based upon a nonlinear diffusion equation. For example, an example of this technique is described in “IEEE SIGNAL PROCESSING MAGAZINE SEPTEMBER 2002” (pp. 16-25).